A BBC investigation has raised concerns about the way the UK's biggest internet child porn inquiry was conducted.

Operation Ore focused on over 7,000 people whose credit cards were used to buy illegal porn from a US website.

Lawyers and computer experts have told BBC Radio 4's The Investigation that many of those arrested may have been innocent victims of credit card fraud.

Police say some on the list may have been fraud victims, but deny that any of them were subsequently prosecuted.

Credit card trail

Lawyers and computer experts said some forces did not carry out proper checks to see if suspects arrested as part of the investigation were fraud victims.

Operation Ore was launched in May 2002 when police received the list with the names of people whose credit cards had been used to buy child pornography from a US website called Landslide Inc.

The police just didn't look for and didn't understand the evidence of wholesale card fraud

Ross AndersonProfessor of security engineering

So far, 2,300 people on the list have been found guilty of offences.

But another 2,000 people spent many months under investigation before charges were dropped.

Ross Anderson, professor of security engineering at Cambridge University, told The Investigation he believed many of those investigated had been innocent victims of fraud.

"The police just didn't look for and didn't understand the evidence of wholesale card fraud," he said.

"And as a result, hundreds of people, possibly in the low thousands of people, have been put through a terrible mill with threats of prosecution for child pornography."

'Fraud victim'

Simon Bunce was one of those arrested after his name came up on the Landslide database.

Nothing was found on his computers and his case was dropped after six months. He is certain he was a fraud victim.

"I investigated diligently myself and I established I was a victim of credit card fraud and identity theft," he said.

The UK's largest police hunt against internet paedophiles was launched after US investigators found customers around the world were accessing images of child abuse from the Texas-based website.

There were cases of credit card fraud but after some investigation they were often found to be just that

Peter SommerLondon School of Economics

Credit card details used to access material on the site gave investigators direct leads on 250,000 people worldwide.

It led to raids at addresses around the UK, involving a number of police forces.

Peter Sommer, a senior research fellow at the London School of Economics who investigated the Landslide site, believes concerns about Operation Ore are "unfounded".

The operation was a success because "hundreds of people were found guilty" and abused children identified, he said.

But Mr Sommer admitted some cases had been dropped because of a lack of evidence.

"During an investigation of this size I would not be surprised if there were some small errors," he added.

"There were cases of credit card fraud but after some investigation they were often found to be just that."

'Most guilty'

Jim Gamble, chief executive of the Child Exploitation and Online Protection Centre and former head of the national crime squad, has been involved with Operation Ore since it was first set up.

He also defended the record of the operation and told the Radio 4 programme that more than 90% of the individuals tracked by police had pleaded guilty.

"That's not about credit card fraud," he said.

"That's people who - the allegation has been levelled against them, the evidence has been collected and they, at court or through accepting an adult caution, which 600-plus of them did, have said I am guilty of this offence."